The recovery of the Arrhenius exponential factor through the conditional probability approach described here, sheds particular light on the nature of molecular energies. From the perspective of reaction paths in a landscape, chemical systems are multistable even in equilibrium and their different components coexist in different wells of stability. In equilibrium, as assumed in the TST, the dominant paths of back and forth reactions are superposable and pass through the saddle points. But out of equilibrium, the global maxima along the dominant paths would no longer be saddle points and would be different for the reciprocal reactions, due to the existence of curls [1]}. Such a dependence of rate constants on the state of the system would seriously complicate the theory. Moreover, the stability of large molecules in equilibrium is itself questionable. Unlike simple eternal molecules, high-energy molecules have average lifetimes that are not infinite, so that considering them in equilibrium may only be an approximation valid at our time scale. The conditional probability perspective greatly simplifies the situation since molecular energies are just energy pawls over which molecular structures are (even if only temporarily) locked. In this view, complex molecules are exponentially rarer than simple molecules.
